Should you replace all four tyres at once?
For most cars, no. Replacing in pairs across the same axle is the standard advice, and replacing a single tyre is perfectly reasonable when the other tyre on that axle is a close match for tread depth, size and type.
There is one significant exception, and it catches people out expensively: four-wheel drive and many all-wheel-drive cars, where the manufacturer often requires all four tyres to stay within a small tread-depth tolerance. More on that below.
The tyre trade has a reputation for pushing full sets. So here is the honest version: a full set is genuinely the right answer in specific situations, and unnecessary in most of the rest. We will tell you which one you are in, including when the answer costs us money.
Why pairs, and not one at a time?
The problem with a single tyre is not the tyre. It is the pairing.
Put a brand new tyre with roughly 8mm of tread next to one at 2mm on the same axle, and the two sides of that axle now have noticeably different grip, particularly in the wet, where tread depth is what clears the water. Under braking, in standing water, or on a wet roundabout, the car will not behave symmetrically.
Across an axle, aim to keep the two tyres:
- the same size, load rating and speed rating;
- the same type, so not one run-flat and one conventional, and not one summer and one winter;
- reasonably close in tread depth.
Different brands front to rear is fine and extremely common. Different brands side to side on the same axle is legal but not ideal.
What is not permitted is mixing tyres of different structure, such as radial and cross-ply, on the same axle. In practice almost every modern tyre is radial, so this rarely comes up outside classics and some trailers.
Why new tyres go on the rear axle
This one surprises people, especially front-wheel-drive drivers who assume the new pair belongs on the driven wheels.
The reasoning is about what happens when grip runs out. If the front loses grip first, the car understeers: it pushes straight on, and most drivers instinctively lift off, which is roughly the right response. If the rear loses grip first, the car oversteers, and correcting that takes a level of car control most people simply do not have when it happens without warning on a wet roundabout.
New tyres have deeper tread and clear standing water better. So the better pair goes on the back, where losing grip is hardest to recover from. This holds for front-wheel-drive cars as well.
That is why a fitter may move your existing tyres to the front and put the new pair on the rear rather than bolting the new tyres where the old ones came off. It is not a trick to add labour; it is the standard safe practice.
The four-wheel-drive rule
If your car is four-wheel drive or all-wheel drive, check the handbook before you replace one tyre.
Many manufacturers specify that all four tyres remain within a small tread-depth tolerance, often a couple of millimetres or less. The reason is mechanical: tyres with different tread depths have slightly different rolling circumferences, so the axles turn at slightly different rates. On an all-wheel-drive system that difference is absorbed by the centre differential or the coupling, continuously, on every mile you drive. Sustained mismatch cooks it.
Replacing a centre diff or a Haldex-style coupling costs several times what a set of tyres costs. If you have an all-wheel-drive car and one tyre is destroyed, that is the moment to check the handbook rather than to save money.
When a full set genuinely is the answer
- All four are near the limit anyway. The legal minimum is 1.6mm across the central three quarters of the tread, all the way round, but grip falls off well before that. If everything is at 2mm, you are replacing the lot within months regardless.
- The car is all-wheel drive and the manufacturer requires matching.
- You are changing type, for example moving off run-flats onto conventional tyres. Do that as a set, not piecemeal.
- The tyres are old. Rubber hardens and cracks with age even with tread left. A tyre is generally worth inspecting closely from around six years old and replacing at ten, and the date is moulded into the sidewall as a four-digit code: week, then year. “3221” means week 32 of 2021.
When it is not
- One tyre has picked up unrepairable damage and the other three are healthy.
- You are on a two-wheel-drive car and the tread depths across the relevant axle are close.
- Somebody is recommending four tyres without having looked at the other three.
Spreading the cost
A full set at short notice is a real amount of money, and tyres never go at a convenient time of the month. We offer interest-free instalments so it can be spread.
That is not a sales line. The reason people drive on tyres they know are finished is almost never that they do not care; it is that the money is not there this week. Spreading it is the difference between a legal tyre now and a fine of up to £2,500 per tyre, plus three penalty points per tyre, later.
Ask us what you actually need
We come to your home, workplace or the roadside across Dundee, Perth, Angus, Perthshire and Fife, 24 hours a day. We will look at all four tyres while we are there, tell you what they have got left, and tell you honestly whether you need one, two or four.
Send your registration for a price or call Matt on 07449 206 581.